In conventional highway-tractor trailer air brake systems, two lines extend from the tractor to the trailer with the tractor having the basic air pressure generating equipment. One line, usually referred to as a trailer "emergency" or "supply" line, maintains constant maximum pressure in a reserve air reservoir mounted on the trailer supplied from a main air reservoir on the tractor. The other line, commonly known as the "control" or "trailer service" line, is pressurized in response to a brake-operating valve on the tractor. The term "rig" is used herein to refer to a tractor and trailer combination as a unit.
The basic conventional brake arrangement on a trailer requires that the trailer brake system be spring-loaded with sufficient spring strength to apply the brakes and bring the entire rig to a stop whenever air pressure in the air motor opposing the spring force to maintin the brake from application drops to a level allowing the pring force to operate the brakes. To implement this arrangement, a first air motor powered by pressure from the trailer air reservoir and controlled by pressure in the trailer supply line constantly opposes the spring-loaded bias of the trailer brakes to maintain released condition of the brakes while pressure exists in this line. A rupture in the "emergency" or "supply" line from the trailer will thus cause exhaustion of the trailer spring set brake chambers and application of the emergency brakes.
A second air motor responsive to pressure in the "control" line to the trailer is mounted in the trailer to apply the trailer service brakes. In this manner, air pressure in the "control" line induced by operation of a foot pedal or other manual control on the tractor results in trailer brake operation.
In the event of rupture of either or both of the two air lines to the trailer, air pressure on the tractor equipment must be sustained at a sufficient level to operate the tractor brakes. Usually a tractor-trailer brake system includes a valve mounted on the tractor dash known to the trade as a "tractor protection control" valve for supplying fluid to a "tractor protection valve" which forwards fluid to the trailer through the trailer "emergency" or "supply" line and opens the trailer "control" line. This valve is manually operated and once opened is held open by pressure in the portion of the emergency line extending to the trailer. Rupture of this line causes no great air loss to the tractor as the valve senses the loss of line pressure and shuts promptly before much leakage to preserve the braking ability of the tractor.
When the "service" or "control" line from the conventionally equipped tractor to the trailer ruptures, a massive air leak occurs when the truck operator attempts to brake the rig and the trailer brakes are prevented from applying because no signal can be transmitted to the trailer.
Leakage will occur until exhaustion of the truck reservoir occurs, or until the "tractor protection control" valve shuts off air flow to the "tractor protection" valve. This vents the trailer supply line, usually after the pressure has dropped to around 40 psi with consequent degrading of tractor brake performance. Meanwhile the trailer brakes do not function until this automatic shut off occurs.
Hence, it is an object of this invention to provide a tractor-trailer brake system in which the tractor brakes remain fully in service after any failure to brake lines between the tractor and the trailer.
It is another object to provide a brake system for bringing the emergency brake on the trailer into operation when the control line thereto fails without substantial pressure loss on the tractor system.
Another object is that, if one of the control lines to the trailer has ruptured and an emergency stop has been made, the entire tractor-trailer brake system will function in a manner to release the trailer brakes and allow movement of the rig to service facilities.